Damping arrangements

ABSTRACT

A method of damping vibrations in the hull of a floating vessel by transferring vibrational energy from the hull to a liquid-gas system within the hull, the vibrational energy being absorbed within the system by compression and expansion of the gas by the liquid and finally dissipated by friction in the liquid-gas system. Assemblies are disclosed for damping vertical, transverse or torsional vibrations in the hull.

United States Patent [151 3,635,182

Paffett [451 Jan. 18, 1972 l DAMPING ARRANGEMENTS [56] References Cited [72] Inventor: James Arthur lhlnel Plflett, Feltham. En UNITED STATES PATENTS land 968.927 8/1910 Frahm ..114/125 [73] Aasignce: National Research Development Corpora- 2,722,194 ll/l955 Hoffmann..... ll4/O.5 X

11611, London,England 2,889,795 6/1959 Parks .I 114/12sx [22] Filed: 1969 Primary Examiner-Trygve M. Blix [2]] Appl. No.: 883,390 Attorney-Camer0n, Kerkam and Sutton [57] ABSTRACT A method of damping vibrations in the hull of a floating vessel [30] Foreign Application Priority Data Dec. 10, 1968 Great Britain ..58,600/68 by transferring vibrational energy from the hull to a liquid gas system within the hull, the vibrational energy being absorbed within the system by compression and expansion of the gas by 58 Field 61 Search ..114/0.5, 125; l8/266, 269, the and mm 1y y he gas system. Assemblies are disclosed for damping vertical, transverse or torsional vibrations in the hull.

iiihl fil s F ar PATENTED JAN 1 8 m2 SHEET 1 0F 5 PATENTED JAN} 81972 SHEET 2 BF 5 PATENTEU JAN I 8 I972 SHEET 3 BF 5 PATENTEUJAWHBYZ 3.635; 182

SHEET 4 [1F 5 GAS RESERVOIR PATENTEU JAN] 81972 SHEET 5 [1F 5 DAMPING ARRANGEMENTS The present invention relates to the damping of vibrations and in particular to the damping of vibrations in the hull of a ship or other floating vessel, e.g., an off-shore oil drilling rig.

These vibrations may be induced in the hull of the vessel by the wave motion of the sea or by operation of the ships propellers. With modern day tankers and bulk carriers of 700 foot length and over, the wave-induced vibrations are compatible in frequency to those of the seas wave motion and resonance occurs. The inherent damping in such ships is low.

According to one aspect of the invention, there is provided a method of damping vibrations in the hull of a floating vessel by transferring vibrational energy from the hull to a liquid-gas system within the hull. Conveniently, the vibrational energy is absorbed within the system by compression and expansion of the gas by the liquid and finally dissipated by friction in the liquid-gas system.

The method may also include the step of changing the gas pressure within the liquid-gas system to tune said system to match the natural frequency of the hull.

According to another aspect of the invention, there is provided an assembly for damping vibrations in the hull of a floating vessel, the assembly comprising a container for a liquid-gas system wherein a pocket of gas is trapped by a free surface of the liquid so that vibration of the hull results in motion of the liquid within the container to expand and compress the gas and dissipate the vibrational energy in the liquid-gas system.

In one embodiment of the invention, the container comprises a tank, a receptacle having at least a lower portion extending into the tank, a closed upper end to the receptacle, and port means permanently connecting the interior of said lower portion with the interior of the tank.

Connection means may be provided for connecting the upper portions of the receptacle and the tank with exterior sources of positive, negative, or atmospheric pressure as the case may be.

The receptacle may be provided by a cylindrical structure extending between the top deck and the base of the hull, the closed upper end of the receptacle being provided by a plate rigidly secured across the structure. Alternatively, the receptacle may be bounded by vertical plates extending between opposite sides of the tank and the closed upper end of the receptacle is provided by a horizontal plate secured between said vertical plates. In this latter instance, the receptacle is preferably provided with imperforate vertical baffles for controlling liquid flow within the receptacle.

Where convenient, the tank for the damping system is provided by a ballast tank or fresh water tank in the hull.

In an alternative embodiment of the invention (for damping transverse vibrations in the hull), the assembly includes a U- tube structure, an upright limb of the U-tube structure providing the container above referred to, and the base limb of the U-tube structure extending substantially from side to side of the hull.

In another embodiment, the container comprises two similar liquid-containing arms each presenting a free liquid surface trapping a pocket of similar gas, the arms and a tank portion of the container joining said anns being arranged in annular fashion in the hull whereby the assembly is responsive to torsional vibrations in the hull.

The invention also includes a hull or vessel using one or more of the various damping assemblies above referred to.

The principal vibrations in the hull may be considered as bending vibrations which may be in the vertical or horizontal planes.

In order that the invention may be more fully understood, embodiments thereof will now be described, by wayof example, with reference to the accompanying drawings of which:

FIG. 1A is a plan view of part of a ships hull illustrating a damping assembly in accordance with a first embodiment of the present invention located amidships of the hull;

FIG. 1B is a plan view of part of a ships hull illustrating a damping assembly in accordance with a second embodiment of the present invention located amidships of the hull;

FIG. 2A is a vertical section of the first embodiment looking forward and taken along the line Il-II of FIG. 1A;

FIG. 2B is a vertical section of the second embodiment looking aft and taken along the line lI-ll of FIG. 18;

FIG. 3 is a diagrammatic illustration of a control system for use with the embodiments of the other figures;

FIG. 4 is a diagrammatic side view of an alternative embodiment of the invention to that shown in FIGS. 1A to 23;

FIG. 5 is a diagrammatic view of a further embodiment of the invention, also suitable for damping transverse vibrations in the hull;

FIG. 6 is a diagrammatic view of another embodiment of the invention for damping torsional vibrations in the hull; and

FIG. 7 illustrates a simplified mathematic model of the hull and its damping system.

The principle of design and operation of the various damping assemblies hereinafter described in accordance with the present invention is illustrated by the mathematical model shown in FIG. 7.

The principal modes of vibration of the hull are the twonode vertical and the twonode horizontal modes in which the vessel bends about two nodal regions spaced apart along the hull. In each of these modes, the vessel presents a respective stiffness K and the vibrational motion of the ship is associated with a respective damping factor C The damping assembly has corresponding properties, the stiffness (K being dependent on the trapped pocket(s) of gas in the liquid-gas system and the damping factor (C being introduced principally by the viscous forces involved as the liquid in the assembly moves over the containing surfaces of the assembly. The effective mass M of the damping assembly is associated with that of the liquid contained (when the assembly liquid is at rest) between the two levels associated with the free surface of the liquid trapping the (or a) pocket of gas and the other liquid surface(s) present in the assembly. This compares with a corresponding mass M for the vessel.

The quantities M,, M etc., will depend on the size and shape of the damping assembly, tank and the gas spaces and the fluids in them. Numerical values may be estimated from experiments, or, in some simple configurations, by calculation. As above indicated the vessel and its damping assembly may be replaced by the mathematical model of FIG. 7 for the purposes of calculating the behavior of the vessel and the assembly. This provides a starting point for designing the damping assembly or a first model of the assembly from which the final version can be arrived at after the necessary trials and experiments have been carried out on the model. Further details may be had from a paper entitled Machinery Induced Vibra' tions and the references given in this paper. This paper is by Johnson and McClimont and it is contained in Volume 75 Number 4 of the transactions of the Institute of Marine Engineers published in April, 1963. Other useful references for this purpose are Mechanical Vibration by .l. P. Den Hartog, McGraw Hill, New York and London, Third Edition I947; Ship Hull Vibration by F. H. Todd, Edward Arnold Ltd., 1961; and a paper entitled 0n the Vibration Amplitudes of Ships Hulls" by Johnson, Ayling and Couchman (1962) to the Institute of Engineers and Shipbuilders in Scotland.

In general it may be said that:

1. As the overall size of the damping assembly is increased frequencies are reduced and forces increased;

2. Frequency is increased by reducing the volume of a gas space, as for example, by lowering its roof;

3. If all the gas spaces are sealed then the frequency may be raised by a uniform increase of pressure throughout the tank;

4. The equivalent suspended mass M may be made large by having tall unbalanced water columns;

5. The damping assembly may be tuned by altering either the total amount of water in the tank, or by altering the differential liquid levels.

The following design considerations must also be borne in mind:

a. All parts of the tank must be accessible for inspection and repair, so passages must be wide enough for a man to get in;

b. The hull or partitions must not be overstressed by raising or reducing pressures;

c. For a new ship the tanks and dampers can be designed together:

(I. For an existing ship the dampers should be designed to be fitted into tanks with a minimum of structural disturbance. Thus narrow tanks to be lowered through holes in the deck are more suitable than large areas of plating.

Referring now to FIGS. 1A and 2A, two such assemblies for damping vibrations in the hull 12 of a floating vessel, are located one on each side of the vessel centerline 14 to provide a symmetrical arrangement. The exact positioning of the assemblies along the length of the hull is a matter of choice and will be decided taking into account all the various factors involved in the building and running of the vessel which in this case is a medium size oil tanker having a number of storage/ballast tanks 16. In the embodiment shown in FIGS. 1A to 2B, the damping assemblies have been positioned amidships of the vessel and this has the advantage that the tanks for the damping liquid may be large so that a relatively large amount of liquid may be used in the damping system to improve its effectiveness. On the other hand if the damping system or systems are located near one end of the vessel (preferably the stern), then the effect of the smaller tanks is at least partially compensated for by the greater displacement of the hull there (and hence of the damping liquid) as the hull bends about its node positions in response to the exciting force.

In more detail the interior of the hull 12 is divided up by bulkheads 18 into a plurality of the tanks 16. When the tanker is carrying little or no cargo, then at least some of these tanks are used to carry water to provide the vessel with the necessaryballast. FIG. 2A shows a vertical section of such a ballast tank modified in accordance with the present invention to provide a liquid-gas damping system for the hull. As will be clear from the earlier description, an identical system is located in the corresponding ballast tank on the other side of centerline l4.

Thus the wing tank 16 shown in FIG. 2A provides the tank of the liquid-gas damping assembly 10 shown in FIG. 2A. The receptacle 20 of this system is bounded at each end by a pair of transverse bulkheads 22 and on each side by a pair of spaced plates 24 extending downwardly from the top deck 26 of the hull and extending from end to end of the tank 16.

At their lower edges 28,'the plates 24 terminate short of the hull bottom to define a slot 30 permanently connecting the interior of the lower portion of the receptacle 20 with the interior of the tank 16. The closed top of receptacle 20 is provided by a plate 32 incorporatingan inspection hatch to allow inspection of the receptacle as and when necessary. Three pipes 34 extending upwardly from plate 32 to above top deck 26 provide connection means whereby the upper portion of receptacle 20 may be connected with an exterior source of positive, negative, or atmospheric pressure as the case may be. Similar (but shorter) pipes 36 are provided for the tank 16. Valves 38, 40 are provided in these pipes so that the receptacle or tank may be sealed 011' or connected up as desired. The assembly structure is completed by two imperforate transverse partition plates 42 located between top plate 32 and the lower edges 28 of side plates 24 and dividing he upper portion of the receptacle into three regions each associated with a respective one of the pipes 34.

A valve 43 (which can be operated from deck 26) allows communication between the tank 16 and the space (44) above the top plate 32. A valve 45 allows space 44 to be opened to the atmosphere if desired.

In the mode of operation illustrated in FIG. 2A, the receptacle 20 is partially filled with water 46 to trap a volume of pressurized air 48 between free water surface 50 and the receptacle top 32. Valve 38 is closed to maintain the pressure of air 48 whilst valves 40 are open to atmosphere so that the water surface 52 in tank 16 is free to assume levels higher than the level of water in the receptacle. Valve 43 is maintained closed. Typically the trapped air is at a pressure in the region of two atmospheres absolute when the assembly is at rest but since the period of any particular mode will depend on the amount of cargo being carried by the vessel at the time, fresh tuning may be advisable if and when this loading is changed.

With the damping assembly arranged thus, vertical vibrations of the hull (such as might be induced by the wave motion of the sea for example) cause vibrational movement of the liquid (water 46) within the assembly 10, the free surface 50 moving up and down within receptacle 20 to alternately expand and compress the trapped air 48. This movement and the frictional factors called into play (e.g., as water in tank 16 moves over the surface of the bulkheads and the ship's side etc.), results in the vibrational energy of the hull being absorbed and dissipated in the damping system 10. It is pointed out here that the partitions 42 play little or no part in the actual damping mechanism of the assembly although they are important in so far as they prevent an end-to-end wave motion being set up in the liquid surface 50. 7

Alternative ways of operating the damping assembly will be apparent to those skilled in the art, it being appreciated that there must be a difference in liquid levels for surfaces 50, 52, in the receptacle and the tank if the system is to respond to vertical vibrations in the hull and permit the associated vibrational energy to be transferred from the hull to the damping system. Thus this difierence may for example be achieved by evacuating the space above the surface 52 and connecting the space above surface 50 to atmosphere. Alternatively both spaces may be pressurized (or evacuated) but with one at a lower pressure than the other. Indeed the illustrated embodiment may be modified if desired by moving the height of top plate 32 so as to have an assembly in which surface 50 may be maintained higher than surface 52. Alternatively where the other of the spaces is initially connected to atmosphere, this space may be sealed off (by closing the appropriate valve) so that its pressure will increase and decrease with vibrations of the liquid 46 in the same way as does the pressure in the trapped pocket earlier referred to. This latter arrangement is capable of giving relatively high operational frequencies in the damping assembly.

If desired, valve 43 may be maintained open to allow the free passage of liquid between the tank 16 and the space 44. This may have the advantage of increasing the mass of damping fluid in the assembly 10 and of utilizing the otherwise wasted space 44 during operation of the assembly. In this case, valve 45 will be operated and connected up in the same way as valves 40 so that the liquid 46 behaves more or less as a single surface extending across the whole area of the tank 16.

FIGS. 18 and 2B show a second embodiment in which the receptacle 20 is now provided by a cylindrical or tubular structure 60 rather than by the plates and bulkheads used in the first embodiment. Corresponding members or parts of members in the two embodiments have been identified in the second embodiment by the same reference numerals as used in the description of the first embodiment wherever possible and details of the purpose, construction, or function of these members may be had if necessary by reference to that earlier description.

The assembly illustrated in FIGS. 1B and 2B lends itself more favorably to the case in which the tanker is to be initially constructed with a view to providing a damping assembly in accordance with the invention rather than that in which an existing tanker is to be modified in accordance with the invention.

In more detail, the structure 60 comprises a cylindrical steel plate assembly extending from the top deck 26 of the hull to the ships bottom 62. The closed top 32 for the receptacle is now provided by a circular plate also incorporating an inspection hatch as in the first embodiment. The connection means mamas between the lower portion of the receptacle and the tank in is this time provided by rectangular ports 64 spaced about the periphery of the receptacle in its lower portion. it will be noted that the partitions 42. of the first embodiment have no counterpart in the second embodiment.

The dimensions of the assembly will depend on the size of hull in which it is employed but typically the tank illustrated would have a height of about 70 feet, a length (axially of the ship) of about 80 feet and a width (transversely of the ship) of about 120 feet. In such a case the diameter of the cylindrical structure 6110 would probably be about 20 feet.

In preparing the assemblies of FIGS, 18 and 2B for operation, valves 3ft, 4'!) are operated to connect the spaces about the liquid surfaces 50, 52 to atmosphere or with appropriate pressure sources (negative or positive as the case may be) in order to provide the required difference in liquid levels between the surfaces 50, 52. The tuning arrangement of FIG. 3 may be used for this purpose but with the plurality of pipes 34 replaced by a pair of such pipes, one for each structure 60. The valves are then closed (unless the space concerned is at atmospheric pressure in which case the valve concerned may be left open if desired) and the system is ready for operation. As before the quantity of liquid (water lb) will have been chosen so that the pressures and liquid levels tune the as sembly to suit the expected vibrations to be damped. The principle of operation of the assembly is as above described with respect to the first embodiment and it will not be further described at this point.

in setting up both the embodiments so far described for operation, water for ballast may be introduced into the tank 16 by an appropriate pump, eg, using ducting MN). The valves 38, ilt) are left open during the introduction of water into the tank to allow the escape of any fumes which may be present as a result of the previous cargo. As already discussed with reference to FIG. 7, the mass of liquid introduced will be critical insofar as this mass will play a part in determining the operational characteristics of the damping system.

The pipes 3 1, 36 are then connected up with the appropriate pressure source or vacuum source (or to at mosphere) to provide a trapped pocket of gas in the system and establish a rating for the system which enables it to be tuned to match the natural frequency of the hull.

A pneumatic tuning arrangement for the assembly of FIGS. 1A and 2A is shoum in FIG. 3 and comprises a compressed gas or air reservoir 70 connected with six pipes 34 (of which only three are shown) for pressurizing the two damping assemblies 10. Pipes 3d are fed in parallel from a common feed branch 72 and each has an associated isolating valve 74! and excess pressure safety valve 76. Pressure in the branch pipe 72 is indicated by a pressure gauge 78. The arrangement is completed by an inlet valve 80 for isolating the reservoir '70 if desired and an outlet valve 32 for exhausting pipes 34 to atmosphere.

If it is found that vertical (and transverse) vibrations in the hull do not exceed acceptable limits when the ship is fully laden, it would be possible to pump the ballast water, e.g., through the ducting 2th), from the tank 16 (and receptacle 20) and replace it by the cargo oil in the usual way. To allow the tank to be fully utilized in this case, the valve 43 is provided in accordance with a preferred feature of the invention to allow passage of cargo oil from the tank into the volume d4 above the receptacle 260. During operation of the tank as part of damping assembly, this volume is of course drained of oil and the valve 43 may be maintained closed, if desired, to prevent the ingress of ballast (damping) liquid into the volume 44.

If significant hull vibrations still occur when the vessel is fully laden, then the cargo oil (in the case of a tanker) may be used as the liquid in the damping assembly. In this case nitrogen or some other inert gas must be chosen for the pocket of gas trapped in the receptacle instead of air. Although the dimensions of the receptacle must be sufficient to take the necessary quantity of gas required for successful operation of the damping assembly, this is not seen to be a serious drawback in view of the other advantages obtained and the fact that at least some of the tanks in an oil tanker are almost invariably something less than full in view of the fact that the ship is usually carrying heavier grades of oil than that for which the total volume of its tanks has been calculated as representing the maximum safe volume to be carried by the tanker.

Modifications of the designs above described will be obvious to those skilled in the art, for example the cylindrical receptacle structure of FIGS. 18 and 2B could be replaced by a rectangular boxlike structure. A more striking variation is shown in lFlG. t which illustrates an embodiment especially suited for use in adapting an existing bull in accordance with the present invention. In this particular embodiment, which is only illustrated diagrammatically in FIG. t, the receptacle takes the form of an inflatable bag 99 of plastics material such as polythene or a rubberized plastics material such as PVC- proofed terylene or rubber reinforced nylon secured within a rectangular tank llo (by tie-cords 92) below the surface 52 of the liquid 46. The bag is substantially circular when viewed in plan. As before, the damping assembly may be located at any convenient position within the hull, e.g., in a central tank of the hull if such tank exists. Alternatively the assembly may be located in a side tank of the hull and if necessary to preserve symmetry, two such assemblies are located on opposite sides of the ships centerline as already described with reference to the earlier embodiments. Thus in one form of this embodiment, the assemblies are located amidships of the vessel with the inflatable bags 90 replacing the tubular structures at of FIGS. M3 and 2Botherwise the damping assembly may be substantially identical to that of the earlier embodiments except that the pipes 34! are now replaced by an appropriate tube 93. This tube may be associated with the tuning arrangement of HG. 3 modified by the replacement of the six pipes 34 (in the FIG. llA-ZA embodiment) by a pair of tubes 93, one for each of the flexible bags 90.

Operation of the flexible bag embodiment is substantially identical to that already described with respect to the earlier embodiments except that the lower end of the bag 90 now carries a mesh 96 for controlling liquid flow into and out of the receptacle. A similar mesh, or a grid, may be provided if desired in the earlier embodiments, e.g., across slot 30 in the first embodiment and somewhere between the rectangular ports 64 and the liquid surface 50 in the second embodiment.

The advantages of using an inflatable bag to adapt an existing ballast tank for operation in accordance with the present invention will be clear to those skilled in the art. These advantages include cheapness, ease of assembly, lightness and easy replacement in the event of undue wear and tear.

So far, the embodiments above described have been designed for damping vertical vibrations. FIG. 5 illustrates diagrammatically a damping assembly which damps transverse horizontal vibrations in the ship, e.g., as introduced into the hull by the ship's engines or the seas wave motion. This assembly varies from the embodiments above described in that the receptacle and tank of these earlier structures are in effect joined together to provide a simple U-shape configuration. The upright receptacle" limb (H20) of the U-tube (W0) as before comprises a closed end (1412) in which a pocket of gas (1148) is trapped by a free liquid surface At its lower end the limb communicates directly with the tank portion of the lU-tube construction provided by the horizontal limb 1116 and second upright limb 21s.

ln the mode of operation illustrated, the trapped gas 1 above the free liquid surface 150 is pressurized and tuned using the FIG. 3 arrangement suitably modified. The other limb 2H6 of the U-tube construction is left open to at mosphere. With such an arrangement the liquid columns in the two limbs 12011, 211% of the tube construction will be of different heights and the assembly will therefore also be capable of damping vertical vibrations of particular frequencies in the hull 12.

In variations of the lFlG. 5 mode of operation, both limbs may be sea ed from the atmosphere if desired and the spaces above the liquid surfaces in the limbs partially evacuated or connected up with appropriate sources of pressurized gas. If it is only desired to damp transverse vibrations in the hull with this assembly, it is not necessary that the two liquid columns should be of different heights (when the assembly is in its rest position). It is, however, necessary that a volume of gas be trapped by a free surface of the liquid so that expansion and compression of the gas can take place and it follows that at least one limb of the U-tube structure must be sealed at its end to contain the pocket of gas.

If necessary to preserve symmetry, e.g., when for any reason a single assembly on the centerline of the vessel cannot be used, then two such stabilizing assemblies can be provided arranged, e.g., in contrary fashion, about the centerline 14 of the hull.

FIG. 6 shows a damping assembly for damping torsional vibrations in the hull 12. The assembly comprises two receptacles 320 in which gas 348 is trapped by the free liquid surfaces 350. The lower portions of the receptacles are connected (through valves 330) with the tank portion 316 of the assembly and as will be clear from the discussion of the earlier embodiments, torsional movement of the hull will cause displacement of the two surfaces 350 to expand and compress the trapped pockets of gas 348 to dissipate the torsional vibrational energy of the hull.

It will be appreciated that the present invention also extends to a hull and a vessel using one or more of the various damping assemblies above described. In particular the FIG. arrangement might be used in conjunction with the FIG. 1A, 2A and/or the FIG. 1B, 2B assemblies.

It should also be noted that the embodiments above described are designed to damp vibrations only of a particular frequency or limited range of frequencies so that separate assemblies will normally be required for each mode of vibration required to be damped. In practice only the two node modes are usually dealt with. However, if for example vibration induced by the engine is especially troublesome, one damping assembly in accordance with the present invention might be positioned adjacent the engine (stem) end of the ship and another damping assembly for damping vibrations induced by the wave motion of the sea might be located amidships or at the bow or stem of the hull.

Although the invention has been described in some detail in its application to a tanker, it will be understood that the damping assemblies of the present invention are equally useful in other types of large vessel. Where these vessels include suitable ballast or fresh water tanks, or even possibly fuel tanks, then the tank or tanks concerned can be utilized to provide the tank of the damping assembly if desired. Alternatively the assembly might be designed as a complete unit for insertion in an existing ship or it might be incorporated into the ship during construction of the ship.

lclaim:

1. An assembly for damping vibrations in the hull of a floating vessel, the assembly comprising a container for a liquid-gas system wherein a pocket of gas is trapped by a free surface of the liquid so that vibration of the hull results in motion of the liquid within the container to expand and compress the gas and dissipate the vibrational energy in the liquid-gas system, said container comprising a tank, a receptacle having at least a lower portion extending into the tank, a closed upper end to the receptacle, and port means permanently connecting the interior of said lower portion with the interior of the tank, the receptacle being provided by a cylindrical structure, the closed upper end of the receptacle being provided by a plate rigidly secured across the structure.

2. An assembly for damping vibrations in the hull of a floating vessel, the assembly comprising a container for a liquid-gas system wherein a pocket of gas is trapped by a free surface of the liquid so that vibration of the hull results in motion of the liquid within the container to expand and compress the gas and dissipate the vibrational energy in the liquid-gas system,

the container comprisinga tank, a receptacle having at least a lower portion extending rnto the tank, a closed upper end to the receptacle, port means permanently connecting the interior of said lower portion with the interior of the tank, and connection means for connecting the upper portions of the receptacle and the tank with an exterior pressure source, the receptacle being bounded by vertical plates extending between opposite sides of the tank and the closed upper end of the receptacle being provided by a horizontal plate secured between said vertical plates.

3. An assembly for damping vibrations in a floating hull, the assembly comprising a tank within the hull containing a constant mass of liquid, means dividing the free surface of the liquid in said tank into at least a first portion and a second portion, means providing an airtight receptacle above one of said free surface portions, and a pocket of gas trapped in said receptacle by said one portion of the free surface, whereby in operation movement of the liquid relative to the tank causes alternate expansion and compression of the trapped gas so that the vibrational energy associated with vibrational distortions of the hull is transferred to the liquid-gas system where it is dissipated principally by work done by the liquid in moving over surfaces provided by the assembly.

4. An assembly as claimed in claim 3 wherein the tank is provided with imperforate vertical baffles for controlling liquid flow within the tank.

5. An assembly as claimed in claim 3 wherein the tank is provided by a ballast tank in the hull.

6. An assembly as claimed in claim 3 wherein the tank is provided by a fresh water tank in the hull.

7. An assembly as claimed in claim 3 including connection means for connecting the receptacle with exterior sources of positive, negative or atmospheric pressure.

8. An assembly as claimed in claim 3 adapted to damp principally vertical vibrations in the floating hull, the assembly including means for establishing the pressure of said trapped gas at a different value, when the liquid is at rest, from the pressure of the gas in contact with the other portions of the free surface, whereby in operation the vibrational energy transferred to the assembly comprises the energy associated with periodic bending of the hull in the vertical mode.

9. An assembly as claimed in claim 8 wherein the free surface of the liquid is divided into said first and second portions by a tubular structure open at its lower end to the liquid in the tank.

10. An assembly as claimed in claim 9 wherein the tubular structure is closed at its upper end to define said receptacle.

11. An assembly as claimed in claim 10 wherein the tubular structure includes port means permanently connecting the interior of its lower portion with the interior of the tank.

12. An assembly as claimed in claim 3 adapted to damp principally transverse vibrations, in which assembly the tank is substantially U-shaped in vertical cross secton so as itself to provide the means for dividing the free liquid surface into said first and second portions, and including means sealing one arm of the U-shaped tank above the free liquid surface portion in that arm to provide said receptacle, whereby in operation the vibrational energy transferred to the assembly comprises the energy associated with periodic bending of the hull in the transverse mode.

13. An assembly as claimed in claim 3 adapted to damp principally torsional vibrations wherein, at rest, the assembly is substantially symmetrical about a central vertical plane, and the tank is substantially annular with its annular axis contained in a vertical plane which is at right angles to said central plane, the substantially annular form of the tank being interrupted at its upper end by two upright terminal tank portions adjacent to and on either side of said central plane, the liquid presenting said free surface portions in said terminal upright portions of the tank.

14. An assembly as claimed in claim 3 in combination with the hull of a floating vessel. 

1. An assembly for damping vibrations in the hull of a floating vessel, the assembly comprising a container for a liquid-gas system wherein a pocket of gas is trapped by a free surface of the liquid so that vibration of the hull results in motion of the liquid within the container to expand and compress the gas and dissipate the vibrational energy in the liquid-gas system, said container comprising a tank, a receptacle having at least a lower portion extending into the tank, a closed upper end to the receptacle, and port means permanently connecting the interior of said lower portion with the interior of the tank, the receptacle being provided by a cylindrical structure, the closed upper end of the receptacle being provided by a plate rigidly secured across the structure.
 2. An assembly for damping vibrations in the hull of a floating vessel, the assembly comprising a container for a liquid-gas system wherein a pocket of gas is trapped by a free surface of the liquid so that vibration of the hull results in motion of the liquid within the container to expand and compress the gas and dissipate the vibrational energy in the liquid-gas system, the container comprising a tank, a receptacle having at least a lower portion extending into the tank, a closed upper end to the receptacle, port means permanently connecting the interior of said lower portion with the interior of the tank, and connection means for connecting the upper portions of the receptacle and the tank with an exterior pressure source, the receptacle being bounded by vertical plates extending between opposite sides of the tank and the closed upper end of the receptacle being provided by a horizontal plate secured between said vertical plates.
 3. An assembly for damping vibrations in a floating hull, the assembly comprising a tank within the hull containing a constant mass of liquid, means dividing the free surface of the liquid in said tank into at least a first portion and a second portion, means providing an airtight receptacle above one of said free surface portions, and a pocket of gas trapped in said receptacle by said one portion of the free surface, whereby in operation movement of the liquid relative to the tank causes alternate expansion and compression of the trapped gas so that the vibrational energy associated with vibrational distortions of the hull is transferred to the liquid-gas system where it is dissipated principally by work done by the liquid in moving over surfaces provided by the assembly.
 4. An assembly as claimed in claim 3 wherein the tank is provided with imperforate vertical baffles for controlling liquid flow within the tank.
 5. An assembly as claimed in claim 3 wherein the tank is provided by a ballast tank in the hull.
 6. An assembly as claimed in claim 3 wherein the tank is provided by a fresh water tank in the hull.
 7. An assembly as claimed in claim 3 including connection means for connecting the receptacle with exterior sources of positive, negative or atmospheric pressure.
 8. An assembly as claimed in claim 3 adapted to damp principally vertical vibrations in the floating hull, the assembly including means for establishing the pressure of said trapped gas at a different value, when the liquid is at rest, from the pressure of the gas in contact with the other portions of the free surface, whereby in operation the vibrational energy transferred to the assembly comprises the energy associated with periodic bending of the hull in the vertical mode.
 9. An assembly as claimed in claim 8 wherein the free surface of the liquid is divided into said first and second portions by a tubular structure open at its lower end to the liquid in the tank.
 10. An assembly as claimed in claim 9 wherein the tubular structure is closed at its upper end to define said receptacle.
 11. An assembly as claimed in claim 10 wherein the tubular structure includes port means permanently connecting the interior of its lower portion with the interior of the tank.
 12. An assembly as claimed in claim 3 adapted to damp principally transverse vibrations, in which assembly the tank is substantially U-shaped in vertical cross secton so as itself to provide the means for dividing the free liquid surface into said first and second portions, and including means sealing one arm of the U-shaped tank above the free liquid surface portion in that arm to provide said receptacle, whereby in operation the vibrational energy transferred to the assembly comprises the energy associated with periodic bending of the hull in the transverse mode.
 13. An assembly as claimed in claim 3 adapted to damp principally torsional vibrations wherein, at rest, the assembly is substantially symmetrical about a central vertical plane, and the tank is substantially annular with its annular axis contained in a vertical plane which is at right angles to said central plane, the substantially annular form of the tank being interrupted at its upper end by two upright terminal tank portions adjacent to and on either side of said central plane, the liquid presenting said free surface portions in said terminal upright portions of the tank.
 14. An assembly as claimed in claim 3 in combination with the hull of a floating vessel. 